The invention relates to all-wheel drive vehicles with a hybrid drive, which comprises an internal combustion engine, a first drive train for a first driven axle, a second drive train for a second driven axle, an electric machine (which may operate either as an engine or as a generator in order to charge an accumulator) in the second drive train, and a first clutch upstream of the electric machine and a second clutch downstream of the electric machine. The first clutch is thus at the internal combustion engine end and the second clutch at the second driven axle end.
A drive of the generic type is known from DE 196 39 904 A1. It permits the following operating modes: driving with electromotive, internal combustion engine-powered or hybrid drive, regenerative braking and charging in the various drive modes, all-wheel drive or drive with only one axle. However, as a result of the fact that the second drive train connects the two drive axles rigidly when the clutches are closed, the situation in the all-wheel drive mode is as if there were no differential between the two axles, and it is therefore not possible to travel around bends.
Further disadvantages of this known drive are due to the fact that the electric machine rotates essentially with the rotational speed of the articulated shaft, and in fact somewhat faster in the variant with a step-down gear mechanism, the rotational speed always being proportional to the travel speed. Thus, during slow travel with the electric drive (that is to say when it is needed the most), sufficient torque is not available and no significant generator effect is available during charging or during regenerative braking when traveling slowly.
DE 44 01 473 A1 discloses a power transmission unit which is arranged in the drive train for the second driven axle, rotates in its entirety and in which a three-element planetary gear mechanism and an electric brake or an eddy current brake are provided. As a result, even though the torque or the rotational speed which is transmitted to the second axle can be regulated, the output variables (torque and/or speed) of this unit can never be greater than the input variables. There is no mention of regenerative braking, let alone of a contribution to the drive.
An object of the invention is to develop a hybrid drive of the generic type in such a way that, as well as improving all the electric functions (drive, charging, regenerative braking), an all-wheel drive which meets all requirements is also provided. These requirements also comprise complete roadworthiness and ABS and ESP compatibility with the permanent all-wheel drive.